4: Is there much scientific
debate over the paleoclimatological records of abrupt climate change?

No one doubts that abrupt climate change has happened
- the evidence for it is overwhelming - but there are considerable controversies
as to where, how, why or even when it happened. The uncertainties bear on the
following points:

Spatial footprints: What is the geographical
extent of a given climate event? Did it occur in both hemispheres? Did
it happen at the same time everywhere, or can we track down its propagation
on the Earth's surface? For example, there is an argument as to whether
the Younger Dryas (YD) cooling occurred in the
Southern Hemisphere as well as the Northern Hemisphere. The main evidence
in the Southern Hemisphere is a glacial advance in New Zealand, the date
of which is unsure. It is a matter of pressing urgency to determine if the
YD occurred in other parts of the Southern Hemisphere and in the Tropics.

Interpretation of climate records: accurate, global coverage of relevant climate variables is very recent,
and in many respects still insufficient. Thus to gain knowledge of the past,
paleoclimatologists must rely on "proxy variables" measured in
various geological objects, such as ice cores, sediment cores (deep-sea
and lakes), tree-rings, corals, etc. Since those are, by nature, indirect
measurements, the exact amplitude of some isolated events is very difficult
to determine. In the colorful prose of the NAS
report: imagine that the climate system is "a device [...] hidden
in a box in a dark room.You have no knowledge of the hand that occasionally
sets things in motion, and you are trying to figure out the system's
behavior on the basis of some old 78-rpm recordings of the muffled sounds
made by the device. Plus, the recordings are badly scratched, so some of
what was recorded is lost or garbled beyond recognition. If you can imagine
this, you have some appreciation of the difficulties of paleoclimate research
and of predicting the results of abrupt changes in the climate system."